Hi,
I think you must have made changes to the /etc/fstab file while booted in aufs mode. If you opened the file and saved it even without making any changes, while booted in aufs mode, then the aufs version of the file takes precedence over the version in the generic partition.
You can solve it "properly" by using the boot menu option to reset the system changes (with or without resetting the user data), but then you will lose any other changes you made in aufs mode.
Or you can workaround it while you continue testing by doing this:
sudo cp /ro/etc/fstab /etc/fstab
( The /ro folder shows you the generic partition in read-only mode, so you can move changes you made there into the aufs mode easily )
But remember to try a full reset of your Ubermix machine before you release it for student use.
On a side note: Have you tried the Xubuntu desktop? I find it almost as fast as Lubuntu, but more mature, more well-rounded. I find Lubuntu has some rough edges for being a full desktop environment. If your concern is about making it look more like Windows, in Xubuntu you can delete the bottom panel, and move the top panel to the bottom. And that's all, it's almost Windows.